Igniter compositions are often used with pyrotechnic compositions in, for example, in small arms tracer rounds. Tracer rounds are typically distributed between intervening non-tracer rounds in a weapon's magazine to allow visual tracking of small arms fire. When a tracer round is fired from a weapon, the igniter composition ignites the pyrotechnic composition leaving a trail, or trace, of the projectile's path. This allows for adjustment of fire to intercept the intended target.
Presently, many Department of Defense small arms pyrotechnic compositions use igniters that require the use of methyl-chloroform (1, 1, 1 trichloroethane) in their manufacture such as I-136, I-194, and I-280 igniters. (I-280 is a mixture of 15% magnesium and 85% I-136). For example, the I-136 igniter comprises a mixture of 90% strontium peroxide oxidizer and 10% calcium resinate binder. The calcium resinate must first be solubilized with methyl-chloroform before the addition of the strontium peroxide. The solubilized calcium resinate coats the strontium peroxide on drying and produces a uniform granular mixture that is dense and free flowing. The calcium resinate also imparts a moisture proof barrier effect that protects the underlying tracer mixture that, being high in magnesium powder, is subject to moisture attack causing malfunction.
I-136 is a dim igniter, that is it emits practically no visible light during the initial part of the tracer projectile's trajectory. This prevents a visible signature, or trace, when the projectile is first fired to avoid blinding the gunner and to prevent an enemy from easily observing the gunner's position. I-136 is used in a broad spectrum of small arms ammunition such as 5.56 mm, 7.62 mm. 50 caliber, and 20 mm rounds that incorporate a tracer element. It may also function as a base for other igniters and tracer compositions and with the addition of varying amounts of magnesium it functions as a subigniter and dull igniter.
I-136 has been in use for 60 years and its manufacture originally used carbon tetrachloride to solvate the calcium resinate before mixing with strontium peroxide to produce a uniform, homogeneous, free flowing powder that was ideal for the loading procedure at ammunition plants. In the late 1960's carbon tetrachloride was found to be a carcinogen and methyl-chloroform was then substituted for the carbon tetrachloride.
However, it has been determined that methyl-chloroform depletes the earth's ozone layer, requiring discontinuation of its use. Additionally, methyl-chloroform is a volatile organic compound that requires volatile organic solvents for clean-up of manufacturing spills. Further, the use of methyl-chloroform requires the use of a solvent recovery system to reduce its toxic effects. However, if strontium peroxide and calcium resinate are dry blended without first solubilizing the calcium resinate with methyl-chloroform, the resulting composition does not have the flowability necessary for the projectile charging procedure and is not water resistant to ensure proper performance under adverse moisture conditions. Moreover, the calcium resinate binder is derived from natural sources with varying purity from lot to lot thus making blending of the proper proportions of calcium resinate and strontium peroxide difficult.
Many attempts have been made without success to produce a substitute I-136 dim igniter without the use of carbon tetrachloride or methyl-chloroform solvents. The pharmaceutical industry's methods in blending dissimilar powders, and other fuel-oxidizer systems that do not require a solvent in their blending were investigated. As examples: (1) ethyl cellulose was substituted for the I-136 calcium resinate to produce a mixture of 10% by weight ethyl cellulose and 90% by weight strontium peroxide. One such mixture was dry blended and another was solubilized with isopropyl alcohol. Each was more difficult to ignite and burned with more flame than standard I-136. (2) Substituting gelatin for calcium resinate was ineffective. (3) Substituting melamine for calcium resinate was ineffective producing a composition that was very difficult to ignite. (4) Substituting carboxy methyl cellulose for calcium resinate in a dry blend produced an undesired bright flame. When the carboxy methyl cellulose was first solubilized with water, the resulting composition failed to ignite because, it is believed, the peroxide lost an oxygen and hence its oxidative potency.
Liquid nitrogen and liquid carbon dioxide (as used to decaffeinate coffee) were considered to eliminate the need to use a solvent recovery system to produce a dim igniter with characteristics similar to the I-136. Other alternative solvent systems were investigated. For example the I-136 strontium peroxide/calcium resinate combination were tested using the following solvents: (1) acetone; (2) isopropyl alcohol; and (3) a 50/50 mixture of acetone and isopropyl alcohol. A 95% strontium peroxide and 5% calcium resinate sample was also tested since the NASA thermodynamic code indicates this is the optimum ratio that produces the highest flame temperature. Both fuzed and precipitated calcium resinate were used in these blends due to their different rates of solubility. Each blend was wetted with each solvent, blended with a spatula and dried over a hot plate. The resulting cakes were passed through a 20 mesh sieve and produced more free flowing compositions that were denser than the dry mixed blends. Additionally, 2 grams of calcium resinate were dissolved in 4 cc of 50/50 acetone/isopropyl alcohol by volume to produce a fluid having the same color and apparent thickness as calcium resinate dissolved in 1, 1, 1, trichloroethane.
None of the above attempts resulted in a satisfactory substitute for the I-136 dim igniter composition.
Accordingly, it is an object of the present invention is to provide a dim igniter composition without the use of carbon tetrachloride, methyl-chloroform or other ozone depleting compounds in its manufacture.
Another object of the present invention to provide a dim igniter composition without the use of volatile organic compounds in its manufacture.
A further object of the present invention is to provide a dim igniter composition consisting of components having consistent purity to allow for efficient, accurate, and uniform blending.
Yet another object of the present invention is to provide a dim igniter composition having the physical characteristics amenable to existing charging procedures at munitions loading plants while having similar performance characteristics as the I-136 dim igniter.
Other objects will appear hereinafter.